1. Field of the Invention
The present invention relates to an optical device and a method for focusing a laser beam. More specifically, the present invention relates to an optical device and a method for focusing a substantially elliptical laser beam, such as for example that one generated by a laser diode.
Preferably, but not exclusively, the device according to the invention is intended for use in an optical reader (for example an optical code reader) in order to increase the laser beam depth of focus.
2. Discussion of Prior Art
As known, an optical code reader is essentially comprised of a reader device, commonly named scanner, comprising a laser beam emitting source (typically a laser diode) intended to illuminate the code to be decoded along a reading line and a light-sensitive element intended to collect the light diffused by the points of such line and generate in turn an analog electrical signal proportional to the light intensity itself. The generated analog signal is then converted into a digital signal, which is processed by a decoding software, which finally provides the sequence of decoded characters of the read optical code.
In order to allow reading and decoding of the optical code, it is necessary that the scanner is further provided with an optical element capable of focusing the laser beam at the distance at which the code to be read is positioned. Typically, such optical element is comprised of a converging lens provided downstream of the laser diode and designed to collect the diverging laser beam emitted from this and focusing it at a predetermined distance; the focused beam is then deflected by means of a rotating polygon (or alternatively by means of an oscillating mirror or by means of an alternate movement of the laser source itself) so to obtain a code scan along the reading line.
Especially in the object handling and sorting plants, there is the problem of being able to read optical codes whose distances and tilts, with respect to the scan line of the scanner, can be very different between each other. In such situations, the use of a laser beam scanner of the above mentioned type not always allows to correctly focus the codes.
Throughout this description and the appended claims, the term: xe2x80x9coptical codexe2x80x9d is used to indicate a code (such as for example a bar code, a two-dimensional code, a color code or the like) containing coded information (for example relative to the objects on which it is provided). Hereinbelow, reference will be made to bar codes, merely in order to make the description more clear.
The inventors of the present invention have studied how to increase the depth of focus of a laser beam emitted by a laser diode through comparative tests on scanners provided with different optical elements and have surprisingly found how such a result is achievable paying specific attention to the form of the laser beam profile.
Hence in a first aspect thereof, the present invention relates to an optical device for focusing a substantially elliptical laser beam, characterized by comprising:
means for collimating a substantially elliptical laser beam so to generate a collimated beam of substantially elliptical profile;
means for circularizing the collimated beam so to generate a beam of substantially circularized profile;
means for focusing the circularized beam.
It has been experimentally proved that it is possible to achieve the best results in terms of increase of depth of focus and overall system efficiency associating the laser diode and the collimating and focusing means with additional means designed for circularizing the collimated beam, so to generate a circularized and confined beam, which shall be then focused on the code to be read.
The use of a laser diode as emitting source is still broadly popular in the field of optical code reading. But such practice, if on one side permits to achieve an advantage in terms of costs, on the other side implies that a diverging beam having elliptical profile shall be emitted.
Advantageously, the presence of the circularizing means allows to overcome the drawback associated to the laser diode scanners of known type, related to the presence of a collimated beam with elliptical profile and therefore of a light footprint having different size on the two perpendicular axis. In fact, as known, the reading of bar codes implies that the footprint size in the reading direction is smaller than the minimum width between those of the narrowest bar and the narrowest space; therefore, an elliptical light footprint could imply the possibility of carrying out an adequate reading only along a determined scan direction. Differently, the presence of a circular beam (and hence of a footprint having the same dimensions in all directions) allows to overcome such drawback, allowing therefore the reading of the codes also when these are tilted with respect to the scan direction of an angle up to 45xc2x0 (tilt angle).
Advantageously, the means for collimating the laser beam comprises, alternatively, a refracting or diffracting lens with a spherical or aspherical profile.
Advantageously, the means for circularizing the collimated beam comprise, alternatively, an anamorphic prism, a pair of anamorphic prisms, a cylindrical mirror tilted of an angle comprised between 0xc2x0 and 90xc2x0 with respect to the propagation direction of the collimated beam, or a pair of cylindrical mirrors, each one tilted of an angle comprised between 0xc2x0 and 90xc2x0 with respect to the propagation direction of the collimated beam.
In further alternative embodiments of the present invention, the means for circularizing the collimated beam comprise a diffraction grating tilted of an angle comprised between 0xc2x0 and 90xc2x0 with respect to the propagation direction of the collimated beam or a pair of diffraction gratings, of which at least one is tilted of an angle comprised between 0xc2x0 and 90xc2x0 with respect to the propagation direction of the collimated beam.
Advantageously, the circularization of the laser beam takes place through an anamorphic enlargement carried out by the aforesaid optical elements. Through a careful design of such elements it is furthermore possible to minimize the residual astigmatism of the laser beam, thus assuring an effective reading on a remarkably broad reading range, also in case of optical codes tilted with respect to the scan direction.
The means for focusing the circularized beam comprise alternatively a converging refractive or diffractive or Fresnel lens, with either a fixed focal length or a multifocal or a slightly toric one.
Advantageously, the use of a focusing diffractive lens in place of a refractive one allows to overcome the drawbacks related to the need of having a lens with very long focal and very small diameter with respect to the focal length, what makes the centering and the control of the precision in the lens manufacture difficult, and hence, at last, the control of the focal length. The intrinsic precision of the method for manufacturing a diffractive lens advantageously allows to control with very high precision both the centering and the focal, and furthermore allows to introduce on the lens an aspherical, toric or multi-focal profile more easily than in case of a refractive lens.
In alternative, the means for focusing the circularized beam comprise a toric mirror tilted of an angle between 0xc2x0 and 90xc2x0 with respect to the propagation direction of the circularized beam.
In further alternative embodiments of the present invention, the means for collimating, circularizing and focusing the beam comprise a pair of cylindrical refractive or diffractive or Fresnel lenses with spherical or aspherical profile, or a single toric refractive or diffractive lens, with spherical or aspherical profile.
The use of toric surfaces advantageously allows to carry out the anamorphic enlargement and the focusing (otherwise effected by the focusing lens) by means of a single optical element.
In a second aspect thereof, the invention relates to a method for focusing a laser beam of substantially elliptical profile, comprising the following steps:
a) collimating a substantially elliptical laser beam in such a way to have a collimated beam of substantially elliptical profile;
b) circularizing the collimated beam so to have a beam of substantially circularized profile;
c) focusing the circularized beam.